Deprecate PromiseProxyMixin, PromiseArray, and PromiseObject

[chriskrycho]

This is blocked on dropping 🔥 DIAF IE11 🔥 support with Ember 4, but once we do, we can begin the deprecation staging process for replacing PromiseProxyMixin, PromiseArray, and PromiseObject. This would have a couple nice knock-on effects:

1. It would remove APIs long known to weird behavior: the way that PromiseProxyMixin and things derived from it try to make the Promise "invisible".
2. It would remove one of the major blockers for removing Mixin in general, as these are some of the key (public API-visible) uses of Mixins in the framework.

Notably, each of these can be straightforwardly (though not necessarily transparently!) replaced by Octane idioms. The PromiseArray and PromiseObject types can likely be reimplemented in terms of native Proxy and also deprecated in favor of less confusing idioms, but doing it that way would allow us to drop PromiseProxyMixin first and then drop those two types later. That will also likely be necessary for making the transition work for Ember Data users.

[runspired]

fwiw PromiseArray and PromiseObject do not come from ember but rather from ember-data and we already deprecated using them directly.

[wagenet]

See #832